In present-day modern power generating systems, such as gas turbine systems, the efficiency plays an important role, because this makes it possible to reduce the costs for operation of the gas turbine systems.
One possible way to improve the efficiency and thus to reduce the operating costs is to increase the inlet temperatures of a combustion gas within a gas turbine.
Ceramic heat insulation layers have been developed for this reason, which are applied to thermally loaded parts which, for example, are composed of superalloys, which on their own could no longer withstand the high inlet temperatures in the long term. The ceramic heat insulation layer offers the advantage of good temperature resistance owing to its ceramic characteristics, and the metallic substrate offers the advantage of good mechanical characteristics in this composite or layer system.
Typically, an adhesion promotion layer composed of MCrAlY (major parts) is applied between the substrate and the ceramic heat insulation layer, with M indicating that a metal composed of nickel, chromium or iron is used.
The composition of these MCrAlY layers may vary, but all the MCrAlY layers are subject to corrosion, despite the ceramic layer on them, due to oxidation, sulfidation, nitridation or other chemical and/or mechanical attacks.
The MCrAlY layer in this case is frequently degraded to a greater extent than the metallic substrate, that is to say the life of the composite system comprising the substrate and layer is governed by the life of the MCrAlY layer.
The MCrAlY intermediate layer is still functional only to a restricted extent after lengthy use while, in contrast, the substrate may still be fully functional.
There is therefore a requirement to reprocess the parts which have become degraded in use, for example turbine blades, guide vanes or combustion chamber parts, in which process the corroded layers or zones of the MCrAlY layer must be removed, in order, possibly, to apply new MCrAlY layers and/or a heat insulation layer once again. The use of existing, used substrates leads to a cost reduction during operation of gas turbine systems.
In this case, care must be taken to ensure that the design of the turbine blades or of the guide vanes is not changed, that is to say that the material is removed from the surface uniformly.
Furthermore, no corrosion products must be left behind which would form a fault source when a MCrAlY layer and/or a ceramic heat insulation layer is coated once again, or which would lead to poor adhesion of the heat insulation layer.
A method for removal of corrosion products is known from U.S. Pat. No. 6,217,668. In this method, the corroded part is accommodated in a large vat, with the part being arranged in a powder bed with an aluminum source. The vat must be partially closed and then heated in an oven. The heating process results in aluminum being supplied to the corroded part, as a result of which the areas can be removed by means of a subsequent acid treatment which would previously not have been able to remove it as well, that is to say it would have had greater resistance to removal.
A large amount of material is required for the powder bed, and the vat occupies a large amount of space in the oven during the heat treatment. The heating process also takes longer, owing to the high heat capacity.
A further method for removal of surface layers from metallic coatings is known from U.S. Pat. No. 6,036,995. In this method, the aluminum source is applied by means of a paste to a corroded part. However, the part must be heated with the paste until the aluminum melts, so that the aluminum does not diffuse into the part until this stage. The melted aluminum layer is difficult to remove, since it adheres to the part very well.